The second decade of life is characterized by an amplified interest in and sensitivity to peers. As a result, adolescents are increasingly susceptible to peer influence, particularly in situations involving risky decision-making. Importantly, evidence indicates that exposure to peers' risky behaviors in online environments contributes to offline risk-taking. Therefore, it becomes possible to investigate the neural correlates of peer influence in an ecologically valid manner, given the feasibility of mimicking social media use in the confines of an MRI scanner. The present project capitalizes on this opportunity by measuring neural responsivity to peer influence and risky images as they are experienced in a social media environment. By using a neuroimaging approach to examine risky decision-making and peer influence, this research can lend unique insight into mechanisms that may buffer individuals from or predispose them towards risky outcomes. Forty-eight adolescents will undergo functional magnetic resonance imaging (fMRI) while completing a task that mimics the social photo-sharing application Instagram. Participants will view a feed of photographs ostensibly submitted by peers, and decide whether to like each image. The proposed study will test how level of peer endorsement of a photograph (indicated by the number of likes each photograph previously received) influences participants' behavior (i.e., whether they like each image) and neural responses while viewing that image. Several images will depict typical risk-taking behaviors, including alcohol and cigarette use. Participants will aso submit their own images for inclusion in the study; this will allow for the examination of the neural correlates of receiving social feedback on one's own online content, and the relation between this response and behavioral measures of susceptibility to peer influence (SPI). Survey measures will assess participants' history of risk-taking, perception of risk, and SPI. The relatio between individual differences in risk-taking/SPI and individual neural and behavioral responses during social media use will be tested. This research will lend insight into both a sociocultural factor (i.e., social media use) and neural mechanisms that contribute to individual risk for alcohol and drug abuse as well as risky sexual behavior, thereby contributing to the development of targeted educational programs designed to reduce youth risk-taking and SPI. In line with the recent efforts of the NIAAA, NIDA, and NCI to form collaborative opportunities for addiction research, the proposed project will investigate behavioral and neurobiological mechanisms that place individuals at increased risk for a range of negative outcomes relating to alcohol and drug abuse during a vulnerable developmental period. In particular, the aims of this project are consistent with the aims of NIAAA's Underage Drinking Research Initiative, which seeks to understand factors that contribute to underage alcohol abuse, including the relation between digital media and underage alcohol use, and the mechanisms whereby social media influences adolescent alcohol consumption.